Low-Valent Ditantalum Complex Ta₂(μ-BH₃)(μ-dmpm)₃(η²-BH₄)₂: First Dinuclear Compound Containing a Bridging BH₃ Group with Direct Ta−B Bonds

The reduction of TaCl5 with LiBH4 in the presence of the bidentate ligand bis(dimethylphosphino)methane, dmpm, produces the novel BH3-bridged ditantalum complex Ta2(μ-BH3)(μ-dmpm)3(η2-BH4)2 (1) and the acid−base adduct dmpm(BH3)2 (2). Two crystalline forms of 1 were obtained, and their structures have been determined by X-ray crystallography. The unique feature of 1 is that the bridging borane is bound only to the ditantalum metal center, forming a simple metallaborane cluster, Ta2B, with averaged Ta−Ta and Ta−B bond distances of 2.7792[5] and 2.307[9] Å, respectively; the averaged Ta−B separation for the axially coordinated BH4- groups, which are bound to the Ta atoms through hydrogen atoms, is ∼0.2 Å longer than that of the bound BH3 group. The μ-BH3 group has one exo-hydrogen atom with a B−Hexo distance of 1.10[8] Å. The distance between the endo-hydrogen atom and the boron atom is 1.23[8] Å. Each of the two endo-hydrogen atoms is also bound to a Ta atom with the Ta−H distance of 1.97[8] Å. Compound 1 represents the first example of a structurally characterized metallaborane complex containing a μ-BH3 group bridging two metal centers. The structure of the air-stable compound 2 has also been determined; it shows a strong P to B dative bond with the P−B distance of 1.903(4) Å.